DEF - a collective term for all the processes by which water in the liquid or solid phase at or near the earth's land surfaces becomes atmospheric water vapor.

the term includes evaporation of liquid water from rivers and lakes, bare soil, and vegative sorces

the term includes transpiration from within leaves and plants

the term includes sublimation from ice and snow surfaces

Field Capacity

DEF - an index of the water content that can be held against the force of gravity.

DEF - water held in the pores after gravity water is taken out

DEF - Water held after a few days of drainage

DEF - Quantity of water held against gravity

ratio of volume of capillary water per unit volume of soilA measurable amount of water per unit of depth

EX) 23% could be considered a capacity to hold 23 cm of water per 1 m of depth

Potential Evapotranspiration

DEF - PET is the rate at which ET would occur from a large area completely and uniformly covered with growing vegetation which has access to an unlimited supply of water, and without advection or heat storage effects.

Many ways to find PET (aka reference-crop ET)

1. T-based

2. Radiation-based

3. Combination

4. Pan

Actual Evapotranspiration

Wilting Point

DEF - when the plant sucks up all of the capillary water, and there is no water left

As long as suction force of plants exceeds capillary force holding the water in the pores, plants will suck up water

As some point, the capillary force exceeds the roots ability to take up water

The percentage of water that cannot be extracted by roots is called the permanent wilting point for that soil

At this point, plants will wilt and if no new water is added, die

In the root zone, soil water in storage can vary from field capacity to wilting point

Vegetation can only use the water that is held between field capacity and the wilting point.

Soils with large differences between the two are generally good for plants

Permeability

DEF - The ability of a material to allow the passage of a liquid, such as water through rocks.

DEF - The capability of a given substance to allow the passage of a fluid.

Permeable materials, such as gravel and sand, allow water to move quickly through them, whereas impermeable material, such as clay, allow water to flow slowly through them

Permeability depends on the size of and the degree of connection among a substance's pores.

Too much, water runs right out

Too little, don't get enough

Vapor Pressure

Water expands greatly.

As a liquid, 1 gram of water takes up 1 milliliter. At 25° C, 1 gram of water occupies about 42,000 milliliters. Molecules are far apart and not able to bond.

Water vapor is like other gases. It can compress and expand and exerts pressure. Water vapor exerts a “partial pressure” which is what we care about now.

The vapor pressure of water changes with temperature and pressure. The volume of a given weight of water vapor varies with temperature and pressure.

Low vapor pressure = low amounts of water in the air, and high evaporation rates

High vapor pressure = high amounts of water in the air, and low evaporation rates

Throughfall

DEF- water that falls to the ground either directly through gaps in the canopy or indirectly having drips off leaves, stems or branches.

Amount of direct throughfall controlled by canopy coverage can be measured using the leaf area index (LAI).

LAI is a ratio of leaf area to ground surface area. When greater than 1, throughfall is smaller. When less than 1, throughfall is greater.

Amount of indirect throughfall also controlled by LAI, in addition to canopy storage capacity, and rainfall characteristics.

high intensity rain means quick throughfall

long duration rain means more throughfall

If area is already wet, there will be quicker throughfall

Interception

Why is it important?

DEF- The process by which precipitation is caught and held by foliage, twigs, and branches of trees, shrubs, and other vegetation, And/or caught and held by human-made structures.

Why important?

1. Some studies show no net loss of water but others show evaporation of intercepted water is greater than evapotranspiration from vegetation

2. For light rainfalls, interception can mean water loss due to E and limited wetting of soil

5. Subtraction of intercepted water from precipitation during heavy rainfalls is insignificant so no effect on major floods

Interception Loss

DEF- The part that is lost by evaporation, never reaching the surface of the ground, is called interception loss.

DEF- Interception loss equals the precipitation on the vegetation minus stemflow and throughfall.

Factors which affect interception loss:

1) Plant physiology

2) Meteorology

Soil

organics/water/mineral matter/air

DEF- a natural body comprised of soils, liquid, and gasses that occurs on the land surface, occupies space, and is characterized by one or both of the following: horizons that are distinguishable from the initial material as a result of additions, losses, transfers, and transformations of energy and matter or the ability to support rooted plants in a natural environment!

goes from just being those components by themselves, to a 'living organism' that can support plant life

The unconsolidated mineral or organic material on the immediate surface of the earth that serves as a natural medium for the growth of land plants.

DEF- The unconsolidated mineral or organic matter on the surface of the earth that has been subjected to and shows effects of genetic and environmental factors of: climate (including water and temperature effects), and macro- and microorganisms, conditioned by relief, acting on parent material over a period of time

A volume of air can hold x amount of water for a given temperature and pressure – relative humidity (It’s relative to temperature and pressure – mainly temperature for us.)

Amount of water that can be held is temperature and pressure dependent.

Relative humidity

how much water the atmosphere can hold at a given temperature.

Saturated air (can’t hold anymore) has a relative humidity of 100%. Less than 100% means the air can hold more water. The higher the temperature, the more moisture the air can hold.

RH describes how close the air is to saturation.

Ratio of actual amount of water vapor in the air to how much the air can hold at a given temperature

Cold air – low capacity

Warm air – high capacity

Relationship of capacity to temperature is not linear

Dew Point

The dewpoint temperature is the temperature at which the air can no longer hold all of its water vapor, and some of the water vapor must condense into liquid water. The dew point is always lower than (or equal to) the air temperature.

If the air temperature cools to the dew point, or if the dew point rises to equal the air temperature, then dew, fog orclouds begin to form. At this point where the dew point temperature equals the air temperature, the relative humidity is 100%.

What factors affect the rate of evaporation?

Be able to briefly explain them.

temperature

vapor pressure

wind

energy available

mixing of water

What factors affect evaporation from the NWS Class A pan?

Most E in real basin or field is from surface of soil or vegetation so water available is limited. (<PE)

Edge effect – E is higher near edge of pan

Water is close to the metal pan

Causes it to be hotter

Caused evaporation to be higher

Pan itself will warm and enhance E

Water in pan is shallow so not much mixing

In a lake, there is circulation going on that you will not get as much in a pan

The heat will not be mixing and spreading around as much

Usually pan will overestimate E so a pan coefficient is used to adjust - 0.70 or 0.75

(about 3/4 of evaporation in the pan is what is going on in a normal environment)

give good estimate of values from a nearby lake or reservoir. Error is about ±15%.

Different Pan coefficients are used for going from pan to crops or dry land.

Pan coefficients are empirical – derived from data and equations based on actual measurements

What do ET processes depend on?

What's on the land surface?

Type of vegetation or surface materials.

Transition Zone into Interception

What are direct and indirect throughfall?

Indirect

Controlled by

1. LAI

2. Canopy storage capacity – amount of water that can be held by the canopy before water starts dripping as indirect throughfall.